In order to make relatively high speed "on-the-fly" thickness measurements on circular disc-like piece parts or articles, it is often desirable to utilize measuring apparatus that is not only precise, but that does not require physical contact with the opposite major surfaces of the articles.
One particular need for such an apparatus, of primary concern herein, is in accurately and rapidly measuring the thickness of disc-like articles employed in telephone receivers. Such articles, hereinafter simply referred to as discs, are initially blanked out of ferrous strip stock material known as chromindur, manufactured by the present assignee, and are often as small as one inch in diameter, with a typical nominal thickness of 0.040 inch. When used in telephone receivers, the discs are ultimately formed into a cup-shape and magnetized. During such processing operations, the initial thickness of the blanked discs is of particular importance. This follows from the fact that if the discs are too thin, the magnetic properties of the ultimately formed cup-shaped receivers may be unacceptable, whereas if the discs are initially too thick, accelerated wear of the forming dies, or even catastrophic die failure, may result. As such, the blanked discs for one particular telephone receiver must have an initial thickness that does not vary by more than .+-.0.002 inch about a nominal thickness of 0.040 inch.
One thickness measuring technique that has been found well suited for the application in question is known as capacitive gauging. This technique involves the use of an opposed pair of non-contacting capacitive displacement probes. Such probes are coupled to an associated measuring circuit which provides an output signal that has a magnitude which is proportional to the thickness of the disc (or any other similarly configured article) being measured.
In addition to affording fast and accurate, non-contacting thickness measurements, a capacitance gauging technique also allows a plurality of measurements to be made at spaced points across the width dimension of each disc. The capacitive gauging probes per se also have the attributes of being relatively durable and stable and, therefore, suitable for even hostile manufacturing environments.
When the discs to be measured are of relatively small diameter, such as on the order of one inch, or less, there has been a need for a greater degree of control over their advancement through the thickness measuring station than can normally be attained by gravity feed. This is particularly the case when a plurality of thickness measurements are to be made at corresponding discretely spaced points on the opposite major surfaces of each disc.
In connection with the need for controlled advancement of each successive disc through the measuring or gauging station, it would also be very advantageous to be able to sense for any predetermined leading and trailing peripheral edge portions thereof. In that manner, reliable START-STOP limits could be readily established to define the maximum time window or period during which one or multiple thickness measurements could be performed on each disc.